The interface of chargers for portable devices such as mobile phones, digital cameras, MP3 players and the like has been integrated into the USB interface, and thus the adapter of the chargers and the USB interface will use the same power source input pin of a connector due to the single power source input pin in the system. Therefore, the system has to detect the input type. As shown in FIG. 1, if the input to a portable device 10 is USB, the portable device 10 can transmit data to and from a host or hub 12 through the data pins D+ and D−. As shown in FIG. 2, if the input to the portable device 10 is adapter, the portable device 10 only can charge a battery. Furthermore, when an adapter 14 is plug-in, the charging current can be set up to 1 A; however, when a USB is plug-in, the charging current will be limited under 500 mA. Therefore, a good method to detect the power source type is important.
According to the USB specification, as shown in FIG. 1, when the portable device 10 is connected to a host or hub 12, each of the data pins D+ and D− at the mater will have a pull-down resistor RDAT—DWN—15K of 15 kΩ connected thereon. Referring to FIG. 2, when the portable device 10 is connected to an adapter 14, the data pins D+ and D− will be shorted to each other. Therefore, the USB detection commonly used is to connect a pull-up resistor RDP—UP—DET of 125 kΩ and a pull-down resistor RDM—DWN—DET of 375 kΩ to the data pins D+ and D− of the portable device 10 respectively, and then identifies the power source type by detecting the voltage levels of the data pins D+ and D−. As shown in FIG. 1, when USB is plug-in, the voltage of the data pin D+ is VDD×[15 k/(125 k+15 k)] and the supply voltage VDD applied to the pull-up resistor RDP—UP—DET is typically 4 V, so the voltage of the data pin D+ is about 0.4 V and is referred to a low level; meanwhile, the voltage of the data pin D− is approximately 0. As shown in FIG. 2, when an adapter 14 is plug-in, the data pins D+ and D− both are at the voltage of VDD×[125 k/(125 k+375 k)], i.e., about 3 V, which is referred to a high level. Thus, the voltage levels of the data pins D+ and D− can be used to identify the power source type of the portable device 10.
However, such detection requires the additional large resistors RDP—UP—DET and RDM—DWN—DET, and the supply voltage VDD is provided by the portable device 10, which implies a burden on the battery of the portable device 10 and even results in false identification when the battery power is low.